Cellular Automaton Simulation of Microstructure Evolution for Friction Stir Blind Riveting

Abstract

Friction stir blind riveting (FSBR) process offers the ability to create highly efficient joints for lightweight metal alloys. During the process, a distinctive gradient microstructure can be generated for the work material near the rivet hole surface due to high-gradient plastic deformation and friction. In this work, discontinuous dynamic recrystallization (dDRX) is found to be the major recrystallization mechanism of aluminum alloy 6111 undergoing FSBR. A cellular automaton (CA) model is developed for the first time to simulate the evolution of microstructure of workpiece material during the dynamic FSBR process by incorporating main microstructure evolution mechanisms, including dislocation dynamics during severe plastic deformation, dynamic recovery, dDRX, and subsequent grain growth. Complex thermomechanical loading conditions during FSBR are obtained using a mesh-free Lagrangian particle-based smooth particle hydrodynamics (SPH) method, and are applied in the CA model to predict the microstructure evolution near the rivet hole. The simulation results in grain structure agree well with the experiments, which indicates that the important characteristics of microstructure evolution during the FSBR process are well captured by the CA model. This study presents a novel numerical approach to model and simulate microstructure evolution undergoing severe plastic deformation processes.

Predicting microstructure evolution for friction stir extrusion using a cellular automaton method

“…As a result, material deformation rate increases leading to a higher strain rate. This trend agreed with other friction stir processes [29,34]. The strain rate around the tool increased by 45% as the rotational speed increased from 180 to 355 rpm (figure 8(d)).…”

“…The friction between the rotating tool and work material also generate a lot of heat during the process, which takes the process temperature beyond the recrystallization point of the work material [36][37][38]. High temperature plastic deformation means the material has conserved enough strain energy to trigger DRX [6,29,39,40]. Three main DRX mechanisms are reported for engineering metals and their alloys, i.e.…”

“…The CA model was built on the evolution of dislocation density. The evolution of dislocation density was modeled based on the authors' previous work [29] for the aluminum alloy for the friction stir blind riveting process. The commencement of DRX at high temperature was modeled using the critical dislocation density approach [19,20,24,28,62].…”

“…On the contrary, FSE process is a high strain rate problem with gradient strain rate from edge to center of the extruded part, which makes it a challenging problem to model the microstructure evolution. In recent years, authors have developed a CA-based microstructure model [29] to simulate DRX governed microstructure evolution of the friction stir blind riveting process of aluminum alloy. Gradient microstructure was simulated at the toolworkpiece interface and validated against experimental results.…”